Field of the Invention
This invention relates to an optical information recording and reproducing apparatus performing a focus and/or track control by normalized focus and/or track error signals.
Recently, there has come into practical use of an optical (optical type) information recording and reproducing apparatus wherein, by condensing laser light and projecting the light onto a recording medium instead of using a magnetic head, the information can be optically recorded and the recorded information can be reproduced.
In the above mentioned optical information recording and reproducing apparatus (there are cases where this apparatus is mentioned just as an apparatus hereinafter), the information of much higher density can be recorded than the case of a magnetic head, and the high densely recorded information can be reproduced by condensing laser light so that the apparatus is expected to come into wide use in the future.
In this apparatus, the information can be high densely recorded and/or reproduced, but on the other hand, unless the tracking which makes a light spot follow on a track, where focusing or information is recorded (or being recorded), so as to bring the light spot projected onto a recording medium under control to become a focusing state, is highly precisely performed, the information to be an object cannot be correctly reproduced.
Because of the adjusting accuracy of an optical pickup or the adjusting condition of a signal detecting system or the like, the above mentioned focusing or tracking generates an offset and there are cases where the accuracy of the focusing or tracking is deteriorated.
In a focusing control, a problem in that a detected error signal (a focus error signal) fluctuates on a large scale is created. This problem results from the structural dispersion which is caused in each performance of the optical pickup and medium related to the detection of error signals. As for the optical pickup, the dispersion of the optical performance of an optical element such as a lens or prism, a mechanical error of a fitting position and an output fluctuation of laser exist, and in the medium, the dispersion of reflectance and the dispersion of pre-groove shape exists. Because of this dispersion, the light intensity and/or the distribution of the light intensity inputting into a photoelectrically converting means detecting an error signal vary and there are problems in that the servo is pulled in a different position from the true focal position due to the generation of offset and in that a stable servo control cannot be carried out due to the wide varying range of an error signal. Therefore, this focusing control will be mentioned as follows. An optical system is adjusted so that the true optical focal point can be detected; however, an optical mechanical adjustment has a limit, which is about 1/10 (.mu.m).
Accordingly, by the focal point shifts by means of the mechanical offset in the optical system, that is to say, by the focus offset, there is a possibility that the servo is carried out on a different position from the true focal position in a state of focusing servo.
Because of this, for example, in the first prior art disclosed in the Gazette of Japanese Patent laid open No. 158046/1983, an offset canceling means in a focusing servo system is disclosed. That is, the sum signal being proportional to the total light intensity obtained in a detecting system is amplified to the offset level of the focus error signal and is input into an adder or subtracter with the focus error signal so that the offset is canceled.
In the meantime, if it is concretely considered that the sensitivity of the detecting system varies in accordance with the reflectance of the recording medium or the fluctuation of the light intensity of the laser diode, 10 to 60 percent of the reflectance and seven to eight times as large as the fluctuation of the light intensity are actually generated so that the sensitivity of the detecting system can vary about fifty times as large as the variation between the sensitivity of the largest case and of the smallest case.
In the above described prior art, if the use of the offset means is restricted only to the reproduction, the means can be materialized; however, in the case of a recording and reproducing apparatus, the light intensity further varies widely between recording and reproducing, therefore, it is necessary to vary the servo gain according to this variation. For example, in the case of reproducing, since the light intensity of laser diode is smaller than the case of the recording (write emitting level), the servo gain for the focus error signal obtained in the detecting system should be made larger enough to supply the servo gain to a focusing actuator.
There are some causes of changing the function of the servo system related to the sensitivity which is chiefly dependent upon the change of the function and others are barely dependent upon the change (a level shift offset) if the servo gain is changed. Therefore, even if the offset can be canceled each other in a state of the servo gain by the method of the prior art, it becomes difficult to compensate the remarkable change of the servo gain and it is practically very difficult to apply stable control to the servo system.
Also, in the second prior art of the Gazette of Japanese Patent laid open No. 78386/1988, positive and negative peak values of an error signal which can be obtained where a servo loop of the servo apparatus is open are detected so that an offset value is calculated from the average value of these peak values, and an offset compensating circuit which subtracts this offset value from the error signal in the case in which the servo loop is closed is disclosed.
In this prior art, in the same manner of the first prior art, a compensating means of the servo gain for the variation (or dispersion) of the sensitivity of a detecting system or the sensitivity of an actuator or the like is not included so that a stable control state can not be established.
Further, in the third prior art of the Gazette of Japanese Patent laid open No. 32264/1985, an optical reproducing apparatus is disclosed. This apparatus keeps a loop gain always constant by subtracting the sum signal being proportional to the total light intensity from a track error signal without depending on the characteristics of the reflectance of a recording medium, the intensity of a light source, an optical detecting element and so forth.
However, in the case of maintaining the apparatus, it is considered to convert an unit as the unit in which the inside of the apparatus is easily maintained. However, for example, in the case in which an optical system is exchanged as an unit, the above mentioned three cases of prior art do not have a means for compensating only the optical deviation included in this optical system. Therefore, a difference in the performance of apparatuses is created according to exchanged units, respectively, if circumstances require, the performance of the constructed apparatus should be evaluated and adjusted, which interferes with the skill performing the maintenance, the number of manufacturing process, a regulator and so forth.